According to U.S. Pat. No. 2,005,730, a mixture of casein, lyes, and/or basic salts and water is introduced into a kneading and extruding device, heat being supplied from the outside to the latter. The initial moisture content is approximately 30%. The product undergoes a relatively small temperature rising during extrusion. The starting product correspondingly has a relatively high moisture content and a pasty consistency. As a result, the extruded product can only be ground with difficulty or not at all and the water solubility is unsatisfactory.
European Patent Specification No. 0021949 describes a corresponding process, in which the intial moisture content is approximately 10%, and a pressure between 15 and 150 kg/cm.sup.2 at temperatures of 30.degree. to 90.degree. C. is applied for between 10 seconds and 1 minute to the mixture of the starting components. As a result of this very low water content, local scorching and brown discolourations can occur. The relatively high residence time under high pressure leads to greater wear and to product damage, whilst the low temperatures can cause incomplete reactions and a lack of product sterilization. In addition, after extrusion, the product is difficult to grind.